Consider this: If you could learn everything important by the age of 14, you could be productive and create new knowledge from then. As more and more people create and increase the amount of knowledge we need, our productive-from date becomes later and later.
So we have to live longer, we have to be alert and creative and cognizant for longer, so we can learn more and become productive later. The alternative is letting the machines remember and think for us, and really I think too far down that road and we'll end up dying off while the computers conquer the stars.
(if you're interested, the author and concept that triggered this was Noam Chomsky and the idea of 'manufactured consent')